Bidentate Lewis Base Adducts of Methyltrioxorhenium(VII) and Their Application in Catalytic Epoxidation

Methyltrioxorhenium(VII) (MTO) forms octahedral adducts with bidentate Lewis bases. These complexes were isolated and fully characterized, including X-ray crystallography. The compounds display distorted octahedral geometry in the solid state with a tendency of disorder concerning the Re central atom. At elevated temperatures, they undergo rapid ligand-exchange reactions in solution. The ease of this ligand exchange depends mainly on the Lewis basicity of the ligand. The more Lewis basic the ligand is, the stronger the metal−ligand interaction is, as can be shown by NMR spectroscopy. All examined complexes are temperature stable but quite sensitive to light and moisture. In the presence of H2O2, the complexes form very active and highly selective epoxidation catalysts. Peroxo complexes are generated, and at least one of the Re−N interactions is cleaved during this process. Total ligand dissociation only occurs in the case of very weakly coordinating bidentate ligands. The peroxo complexes of the MTO Lewis base adducts are, in general, more sensitive to water than MTO itself.